Earthworms of the Amazon (Taxon of the Week: Urobenus buritis)

This week's Taxon of the Week post has been delayed a little: getting hold of some of the reference for it required me to enter a real library and locate an actual journal physically printed on paper. Always an experience.

Urobenus buritis (no pictures of this one, I'm afraid) is an earthworm only recorded from the region of Manaus in the state of Amazonas in northern Brazil. This may or may not be significant. Perhaps not surprisingly, collections of earthworms around the Amazon have been somewhat scattered; most of our knowledge of the area (including U. buritis itself in Righi et al., 1976) can be credited to the work of one researcher, Gliberto Righi. Nevertheless, what we do know suggests that many species in the area have very localised ranges (James & Brown, 2006). Earthworms are poor dispersers and many of them are very particular in their habitat preferences, making them highly vulnerable to disturbance.

The genus Urobenus possesses three pairs of calciferous glands in segments VII-IX (many other glossoscolecid genera possess only two*) and can be distinguished from the similar genus Rhinodrilus by the shape of the glands. In Rhinodrilus, all three pairs of glands are tubular; in Urobenus, the first two pairs are tubular and the third pair (in segment IX) is sac-shaped (Righi, 1985). Urobenus buritis has three pairs of spermathecae, delicate anterior septa and the male pores opening at segments 20-21 (U. brasiliensis, which U. buritis was originally described as a subspecies of, has them at segments 19-20).

*Offhand, this is something to consider when thinking about how vertebrate diversity compares to invertebrate diversity. Two externally quite similar taxa, that the vast majority of people would regard as both being 'just worms' and pretty much identical, may actually differ in something as seemingly basic as how many organs they have.

I haven't seen the original description of Urobenus buritis, so I can't say whether there is any information on its lifestyle, but the closely related U. brasiliensis is an epigeic (living above the ground) species inhabiting leaf litter (James & Brown, 2006). Many of the earthworms around the Amazon have cyclical life histories to deal with the contrast between wet and dry seasons. Tuiba dianae migrates towards drier forest as flood waters rise, maintaining a distance of at least five metres from the water's edge. Other species, such as Andiorrhinus tarumanis, climb up the nearest tree during the wet season and take up residence in patches of leaf litter trapped in the forest canopy while the ground is flooded. Whether Urobenus buritis indulges in such behaviour is currently unknown.

5 comments:

getting hold of some of the reference for it required me to enter a real library and locate an actual journal physically printed on paper

I, for one, appreciate the lengths you are willing to go to for providing us with information.

Earthworms are poor dispersers

Is that really true in general? I thought they, or at least their eggs, are actually rather good dispersers. For instance, at least one species of earthworm has managed to reach (though perhaps not establish itself on) the island of Surtsey. Are there any studies on how widely spread earthworms are on oceanic islands?

Other species, such as Andiorrhinus tarumanis, climb up the nearest tree during the wet season and take up residence in patches of leaf litter trapped in the forest canopy while the ground is flooded.

Earthworms that climb trees? Considering that rainforest leeches also climb trees I guess I shouldn't be too surprised to learn that, but still...

I thought they, or at least their eggs, are actually rather good dispersers.

The impression that I got while looking stuff up was that most earthworm species have fairly small ranges except for a small number of so-called 'peregrine' species. Only one glossoscolecid qualifies as a peregrine, Pontoscolex corethrurus (which James & Brown say is possibly the world's most abundant earthworm). I don't know what distinguishes the peregrines from other earthworm species though I presume it relates to their ability to withstand adverse factors. Certainly P. corethrurus rapidly outcompetes other glossoscolecids in disturbed habitat in Brazil while other peregrines are elbowing out native earthworms in various places around the world.

Earthworms that climb trees?

The description of their climbing in James & Brown is rather neat: "they extend the anterior part of the body and remain still until the secreted mucus binds to the trunk, after which they retract the posterior part, which subsequently adheres to the trunk and allows the following upward movement of the anterior portion. Keeping the body in an 'S-shaped' form seemed to facilitate upward movement. If the individuals were mechanically disturbed, they fell to the forest floor. The animals move upwards only at night, when the high nocturnal humidity prevents desiccation. During the day, the animals hide in moist places along the trunk, such as under the bark. When the waters recede they descend to the ground and resume life in the soil-litter interface".

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I'm an entomologist and taxonomist, currently based in Perth, Western Australia. If you'd like to comment (or offer work), I can be e-mailed at gerarus at westnet.com.au.Subscribe to Catalogue of Organisms